https://github.com/apertium/apertium/pull/181

From 08578745ba3988ece9596eaca734d08e64bae4e1 Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Sun, 18 Dec 2022 03:39:58 +0000
Subject: [PATCH] configure.ac: fix bashism
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configure scripts need to be runnable with a POSIX-compliant /bin/sh.

On many (but not all!) systems, /bin/sh is provided by Bash, so errors
like this aren't spotted. Notably Debian defaults to /bin/sh provided
by dash which doesn't tolerate such bashisms as '=='.

This retains compatibility with bash.

Signed-off-by: Sam James <sam@gentoo.org>
--- a/configure.ac
+++ b/configure.ac
@@ -95,7 +95,7 @@ for version in 23 2b 20 2a 17; do
   version_flag="-std=c++${version}"
   AX_CHECK_COMPILE_FLAG([${version_flag}], [break], [version_flag=none])
 done
-AS_IF([test "$version_flag" == none], [
+AS_IF([test "$version_flag" = none], [
   AC_MSG_ERROR([Could not enable at least C++17 - upgrade your compiler])
 ])
 CXXFLAGS="$CXXFLAGS ${version_flag}"